Within telecommunications systems, data has traditionally been distributed using Time Division Multiplexing (TDM). This is true for both trunk links, e.g. connecting exchanges, and within network nodes such as gateway nodes interconnecting networks. In a TDM system operating at a given Bode rate, a number of channels are defined by allocating specific slots within successive time frames to given channels.
FIG. 1 illustrates the structure of an exemplary gateway of a telecommunications system (the gateway may be entirely contained with a single rack, or may be geographically distributed). Four line-input cards 1a-1d receive data from a set of conventional telecommunication links, e.g. T1 or E1, whilst an uplink card 2 is coupled to a further conventional telecommunication link operating at a higher data rate, e.g. a T3, E3, SONET or SDH link. The gateway also contains a number of central resource “pools”, i.e. a modem pool 3, remote access concentrators 4, and Voice over IP (VoIP) codecs 5. Other pools such as an echo cancellation pool may be provided. Data is distributed between the line cards 1, the uplink card 2, and the resource pools 3 to 5 via a TDM backplane 6 (e.g. H.110). A typical TDM backplane may provide 4096 channels, each of which can carry data corresponding to a telephone call.
In a TDM system, it is critical that components are able to operate in synchrony with one another. Components must be able to place data on, and take data off, the TDM backplane at the correct points in time. The TDM backplane therefore carries a common clock signal, driven from an appropriate reference clock. All TDM components synchronise to this common clock line. In FIG. 1, the reference clock signal is received by the uplink card 2, and is placed on a common clock line (which in the Figure is shown integrated into the TDM backplane 6).
TDM based systems have a number of disadvantages. In particular, such systems do not scale well to high channel counts as higher channel counts require a proportionately higher clock frequency. Also, in order to retrieve the correct data from the backplane, the phase of the clock signal must be matched at each point along the backplane. Therefore it is not straightforward to couple multiple racks together given tight phase constraint and high frequency requirements on the common system clock line. For these reasons, operators and equipment manufacturers are moving to packet-based backplane systems in which the TDM backplane is replaced by a packet network backplane. This is illustrated in FIG. 2, with the packet backplane being indicated by the reference numeral 7. An example of a suitable packet backplane 7 is an Ethernet-based backplane. Such a system has a number of advantages. In particular, the system is easily scalable as the backplane can be extended between system elements with a simple Unshielded Twisted Pair (UTP) cable, and there is no limit placed on channel count. In addition, network hardware for packet networks is relatively cheap and readily available, as compared to TDM hardware.
As with systems using a TDM backplane, within a packet backplane it is necessary to synchronise the clocks of components coupled to the backplane. A consequence of any long-term mismatch in the clock frequencies of components is that the packet queues within individual cards will fill up or empty depending on whether the receiving component clock is running slower or faster than the transmitting component clock. This results in the loss of data and degradation of the service. However, the advantage of a packet backplane over a TDM backplane is that with the former it is only the mean frequency that must be matched, not the phase of the clock.
One solution to the synchronisation problem is that used in TDM backplane based systems, i.e. connecting all components to a common clock line. However, it is desirable to be able to move towards a system in which all components are interconnected only via a packet network, and to avoid the need to connect components to a common clock line. This is particularly so in large systems or in systems where the system components are not contained within a single rack and are perhaps spread over a relatively wide area.
Mechanisms for synchronising computers coupled to a network, e.g. the Internet, are known. For example, the Network Time Protocol (NTP) provides a means for synchronising computers to a common time reference and involves the sending of time “stamps” from network servers to client computers. Round trip times are measured between servers in order to determine the time offsets between servers. NTP can provide control of a clock to within a few milliseconds. This is not sufficient for a low latency packet backplane.
It is an object of the present invention to provide a mechanism for synchronising clocks over a packet network. This object is achieved by broadcasting or multicasting timing signals from a master clock, over the packet network, to other slave clocks.